Sky & Telescope - December 2023 - 8

NEWS NOTES
As the Martian mud dried out, it
shrank and fractured into T-shape junctions,
which repeated wet-dry cycles
softened into a Y shape. Where several
of these junctions meet, they create
the distinctive patchwork of polygonal
cracks that Curiosity saw.
The mud cracks date to the Noachian-Hesperian
transition 3.8 to 3.6
billion years ago. The pattern may have
emerged as Gale Crater was repeatedly
flooded and/or as ground water swelled
upwards. Rapin and his team think that
a salty sulfate crust running along the
cracks' edges has preserved their shapes
over billions of years.
" We know that wet-dry cycles can
SOLAR SYSTEM
What Mud Cracks Mean for Life on Mars
The rover team found the cracks
PLANETARY SCIENTISTS analyzing
data from NASA's Curiosity rover have
spotted evidence of sustained wet-dry
cycles on early Mars. The find is intriguing
given that similar climatic cycles
have long been linked to the advent of
life on Earth.
In the August 10th Nature, a team
led by William Rapin (University of
Toulouse, France) describes mud cracks
that Curiosity encountered in 2021.
" These particular mud cracks form
when wet-dry conditions occur repeatedly
- perhaps seasonally, " Rapin says.
pCuriosity's MastCam captured this panorama
of mud cracks on Mars. Polygonal shapes form
after multiple wet-dry cycles.
while ascending Mount Sharp, the
central peak that towers 5.5 kilometers
(3.4 miles) over Gale Crater. The rover
examined cracks it encountered near a
rock nicknamed " Pontours, " which lies
in a transitional region between a clayrich
layer and another layer enriched
with salty sulfates. Clay-rich layers tend
to form in water, whereas salty layers
emerge when water dries up.
drive chemical reactions to obtain the
building blocks of life, " says Sidney
Becker (Max Planck Institute of Molecular
Physiology, Germany), who was
not involved in the research. " Finding
those conditions on Mars is an exciting
discovery. " That's because as the water
begins to dry up, the concentration
of soluble ingredients in the remaining
water increases. This heightened
concentration can then boost chemicalreaction
rates and raise the chances of
constructing the complex molecules on
which life relies.
However, Becker also points out that,
even if there was wet-dry cycling, we
don't know if Mars had the right atmospheric
or mineral ingredients for life.
And even meeting those requirements is
no guarantee. " The conditions needed
for the origin of life might be different
to the ones that actually create the
needed building blocks, " Becker says.
If wet-dry cycles did help create
ancient life on Mars, they could also
have worked against it. " The conditions
to sustain life over a long period
of time again could be very different, "
Becker says. " Since first life was likely
very fragile, wet-dry cycling might have
caused too much external disturbance. "
Life's maker could ultimately have been
its destroyer.
So, while these cracks are an impor10
cm
pA close-up of mud cracks alongside the same image with the polygonal shapes outlined in red.
The hexagons average 4 centimeters (1.5 inches) across.
8
DECEMBER 2023 * SKY & TELESCOPE
tant piece of the puzzle, we are still a
long way from being able to say whether
ancient Mars hosted life.
¢COLIN STUART
NASA / JPL-CALTECH / MSSS / IRAP (2)

Sky & Telescope - December 2023

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